Silicone plastics are commonly produced by reacting organosiloxane prepolymers or polymers in the presence of a catalyst. The organosiloxane polymer may correspond to the general formula ##STR1## where x is an integer having a suitable value or range of values so that the polymer is a viscous liquid, and whose R, R.sup.1, R.sup.2, and R.sup.3 are organic radicals independently selected from lower alkyl radicals and aryl radicals. The terminals hydroxyl groups may be replaced by other functional groups such as lower alkoxy (i.e. alkyl - blocked hydroxy to increase the storage stability or pot life of the polymer), hydrogen, vinyl etc. In the presence of an appropriate catalyst, functional groups on the polymers react together with elimination of water, or other appropriate end group condensation or addition reaction, to form high molecular weight polyorganosiloxane resin, thus: ##STR2## The resulting high molecular weight resins may be derived from a single organosiloxane by homopolymerization thereof, or by copolymerization of two or more organosilioxanes. The molecular weight of such silicone resins can vary over a wide range. Cross-linked resins may also be formed in this way.
It is also known to prepare foamed plastics of polyorganosiloxanes. This is accomplished by including in the polymerization mixture an organosiloxane polymer which, on condensation polymerization, releases gaseous hydrogen in situ, to act as blowing agent as the condensation polymerization proceeds. Such a compound may have the same general formula as (I) above, but in which a hydroxyl group is replaced with hydrogen. ##STR3## Thus silicone foams are created, with the gaseous hydrogen generated in the reaction blowing the polymerizable mixture.
Silicone foams thus prepared and having this general structure tend to be deficient in mechanical properties, especially tensile strength, flexual strength and tear strength. In order to improve these properties, it is known to provide in a silicone rubber or silicone foam a small amount of carbon-carbon bonding, e.g. by use of vinyl-terminated silicone polymers. Thus silicone rubber or silicone foam prepared from a polymer having vinyl functional groups and polymerized with organohydrogen-siloxanes, using platinum or palladium compound catalysts, has much improved physical properties, thus: ##STR4## Foams can be prepared from such resins if a small amount of water and a certain excess of the hydrosiloxane (organohydrogensiloxane) is present - this is shown, for example, in U.S. Pat. No. 4,189,545 Modic. The reaction is generally as follows: ##STR5##
The siloxane polymeric backbone chain has a natural fire resistance. Organic substituents thereon tend to reduce the fire resistance. Accordingly, it is common to incorporate inorganic filler such as aluminum oxide, silicon dioxide, silicates, etc., into polyorganosiloxanes to obtain improvements in the fire resistance of the flexible foams thereof.
Prior art silicone foams have been limited in their utility by their relatively high density. The minimum density of silicone foam previously achieved has been about 15 lbs per cubic foot (pcf) and commonly such foams have a density in excess of 20 pcf. This restricts their uses to applications such as fire resistant insulation and fire barriers in static structures capable of withstanding heavy loads, and in electrical insulation. They cannot be used in practice in applications such as vehicle insulation, vehicle upholstery, or in aircraft, where their high degree of fire resistance and non-toxicity on subjection to heat would otherwise make them very attractive.
It is an object of the present invention to provide silicone foam from resins with vinyl unsaturation, of low density, along with processes for their preparation.
It is a further object to provide such low density silicone flexible foams which have a high degree of fire resistance, very low toxicity and acceptable mechanical properties.